The invention is generally related to the art of electrically resistive heating elements, and more particularly to the process art of creating an electrically resistive heating element on a structural element, such that the surface of the structural element heats and radiates infrared energy.
The invention employs the use of an arc-sprayer, in a series of steps, to create a resistive electrical component, wherein the resistive component is generally supported by a structural element, such that when the resistive component is carrying an electric current, it heats producing a thermally radiant surface. Arc-sprayers are commonly used in the manufacture of casting molds to line the inner walls of the casting mold with a metal, which greatly improves the durability of the mold. An arc-sprayer produces an atomized metal spray by liquefying a continuously fed metal wire in an electric arc and then atomizing the molten metal with a pressurized stream of gas, which throttles out into a prescribed spray pattern. Arc-sprayers are reported to be the method of choice for applying a purely metal coating to plastic materials such as ABS, vinyl, polypropylene and urethanes; because an arc-sprayer, in comparison to a torch-sprayer, produces a relatively cooler spray. In general, the metals that are used with an arc-sprayer are selected for the properties they impart to the casting mold. In decorative applications, such as bronzing a statue, color can predominate. In order to be suitable for application via an arc-sprayer the metal must be conductive, however, the conductivity of the applied coating is usually not a consideration in selecting a preferred metal, as durability or color requisites dominate. Zinc, and alloys of zinc such as Kirksite, and copper are often the metals of choice. Arc-spayed zinc has been reported to be suitable for producing EMI shields for co-axial cable, and computer and video cabinets. Copper, which of course is a very common conductor, is only about 3.6 times more conductive than zinc. Copper forms a hotter spray than zinc, and has been used less frequently on plastics for EMI shields. Zinc at 5.92.times.10.sup.-6 ohm cm has a volume resistivity that is about 20 times that of nichrome, which is commonly used for wire heating elements, while zinc is generally considered too conductive to be used for heating elements.
In the instant invention, arc-sprayed zinc, applied in a series of process steps, has been found to be suitable for producing large dimensioned, low voltage heating elements. Further, the volume resistivity for the zinc in these heating elements is about one hundred times higher than is expected, which has unanticipated benefits. The heating elements are formed as an integral functional component of the targets. The targets are used in combination with infrared detectors, to practice detection, recognition and destruction of military targets, which are usually equipment and personnel. The heating elements are designed to simulate the thermal image signatory of the real-life versions. Infrared emitting targets must meet a number of criteria to optimize their utility, and the instant invention is amenable to creating thermal images on widely differing target sizes, shapes and performance characteristics.
The surface of the target must emit a quantity of heat that imitates real life equipment and personnel. A temperature 15-50 degrees Fahrenheit above ambient is usually sought. The surface must attain this temperature in a matter of seconds after the target is activated. The size, shape and distribution of the irradiating thermal image must mimic the real life counterpart. The cost of construction of the total target must be relatively inexpensive, as destruction is the ultimate goal. The target must have good weatherability as it will be used outdoors. In is very important that the target be capable of withstanding several hits and still maintain its thermal image. The target should be safe to operate with respect to the auxiliary equipment (generators and batteries) as well as the supporting personnel.
In Prosser's co-pending patent application entitled "Thermal Integrated Targets" filed Sep. 10, 1990, Ser. No. 579,619, a method is disclosed for making thermally radiant targets using metal filled coatings to create a heating element or resistive coating as it is referred to in the application. The instant invention contains features that would be applicable to the same method, albeit with an arc-sprayed metal resistive coating.